The field of art to which this invention pertains is cyanate esters and catalysts for curing such esters.
Cyanate esters, which are described in U.S. Pat. No. 3,553,244, are prepared by reacting polyhydric phenols with cyanogen chloride. Such cyanate esters can be cured by heat alone, but, preferably, are cured by using a catalyst plus heat to form thermoset resins useful as laminating and molding resins. In U.S. Pat. No. 3,962,184, the use of zinc octoate, catechol and triethylenediamine as catalysts for cyanate esters is described. This patent also describes the use of imidazoles alone or combined with organic metal salts, such as zinc octoate, tin octoate, tetrabutyl ester of titanic acid, zinc stearate, tin stearate, or calcium stearate, and phenolic compounds, such as phenol or catechol.
Other references which disclose metal salts and/or aromatic hydroxy compounds as catalysts for cyanate esters are U.S. Pat. Nos. 4,026,913, 4,110,367, 4,195,132, 4,429,112, 4,330,658 and 4,330,669.
U.S. Pat. No. 3,694,410 discloses that chelates, such as iron, cobalt, zinc, copper and manganese acetylacetonates, are useful to catalyze the cure of cyanate esters. The patent, however, does not disclose that metal acetylacetonates can be dissolved in an alkylphenol to form a liquid catalyst solution.
U.S. Pat. No. 4,604,452, discloses that metal carboxylates may be dissolved in alkylphenols to form a catalyst solution that can be added to polycyanate esters of polyhydric phenols to facilitate curing. Although a metal carboxylate of acetoacetic acid is disclosed, metal acetylacetonates are not disclosed as suitable catalysts.
Metal salts, and chelates, when used as catalysts, do not readily dissolve in undiluted cyanate esters. They can form insoluble or gel-encrusted coated droplets which remain as undesirable particulates in the cured compositions. Also such incompletely soluble catalysts require high temperatures and/or long cure times in order to complete the cure. Complete cures are obtained when substantially all of the cyanate ester groups are reacted to form triazine ring structures. Compositions which are incompletely cured will whiten, blister, become embrittled, or even soften to a slime consistency when exposed to steam over a period of time.